PROJECT SUMMARY/ABSTRACT Transposable elements (TEs) are the most abundant sequences in the human genome. However, often dismissed as junk DNA, their roles in genomic structure and function remain poorly understood. Among all TEs, LINE-1s (or L1s) are the most prevalent by mass, constituting 17% and 19% of the mouse and human genomes, respectively. Multiple characteristics make L1s central to our understanding of human TEs. The objective of this R15 application is to define the developmental timing of L1 retrotransposition, which is key to our understanding of L1-mediated mutational load. Recent literature supports the notion that L1 retrotransposition occurs predominantly during embryonic development. However, L1 transgenic models used previously may be inadequate for modeling the developmental timing of endogenous L1s. We have developed a new single-copy L1 transgene model that faithfully recapitulates endogenous regulatory mechanisms. We will use this transgenic mouse model to test a novel two-pronged hypothesis about the tempo and mode of retrotransposition during development. Aim 1. Determine the tempo and mode of L1 retrotransposition during normal development. Aim 2. Determine the scope of L1 retrotransposition under compromised L1 regulation. We expect the proposed research, once completed, will have a major impact on our current understanding of L1 biology. We predict an unbalanced distribution of retrotransposition mutational load in a population. A disproportionally large number of insertions are expected in an ?exceptional? subset of individuals who have relaxed control of their retrotransposons. The knowledge gained will help to explain retrotransposition-derived structure variations and to assess the role of retrotransposition in health and disease, such as cancer and neurodevelopmental disorders. As part of the AREA program, the project will also provide authentic hands-on research opportunities to undergraduate and graduate students, and help to strengthen the research environment at PI?s home institution.